Question: how was water brought to the earth?

David Pyle answered on 23 Mar 2011:

Some of it probably came in on comets!

The total composition of Earth looks similar to a class of meteorites called C1 Chondrites. These meteorites are rich in C, H as well as silicate materials, and iron. So it is quite likely that as the Earth grew from many small collisions of these meteorites, it would have started off with quite a lot of C and H. The Earth is quite likely then to have got very hot – both from radioactive heating, and gravitational heating as the planet began to contract and get denser. As it heated up, lots of ‘volatile’ species, like water, would have been driven off and either lost in space, or held in a primitive (early) atmosphere. About 800 million years into its history, Earth suffered a huge jolt in a collision with a large planetary body that would have burnt off the early atmosphere, and led to the formation of the moon. So Earth should probably be more dry than it is. Where might the extra water have come from? It is certainly plausible that extra water has been delivered to earth on comets, over time. The comet collision rate with Earth would certainly have been large enough in the early solar system, and would have dropped off a lot over the past couple of billion years.